1. Field
The present invention relates to a coherent wavelength tunable laser apparatus, and more particularly, to a coherent wavelength tunable laser apparatus, which is capable of generating an external cavity wavelength tunable laser, modulating a phase of the generated laser, and being formed in a single package form.
2. Discussion of Related Art
Currently, an optical communication system, such as a Dense Wavelength Division Multiplexing (DWDM) optical communication system, adopts an amplitude modulation method of determining whether an optical signal is present and determining a result of the determination as 1 or 0 as a high speed optical modulation method. The amplitude modulation method modulates an optical signal output from a laser device by using an external Electro-Absorption Modulator (EAM) or an external Mach-Zhender modulator. However, the amplitude modulation method has a problem in that a modulation speed is increased, a width of a laser spectrum is also increased. The increase in the width of the laser spectrum has a disadvantage in that noise power is increased, and a range of full speed is limited due to a problem of color distribution of an optical fiber.
In order to overcome the disadvantage of the amplitude modulation method, a phase modulation method is introduced in order to improve a transmission speed. The most representative method of the phase modulation methods is Differential Phase Shift Keying (DPSK) of determining phases of 0° and 180° and determining the determined phase as 1 or 0. The DPSK using a difference between the two phases has the same transmission speed as that of the amplitude modulation. However, when Differential Quadrature Phase Shift Keying (DQPSK) of increasing a transmission speed by two times by making a phase difference have 90° is introduced, there is an advantage in that the transmission speed may be increased by two times at the same modulation speed, but an external optical modulator formed of two or more Mach-Zhender modulators is required in order to modulate a phase.
In the meantime, a laser is configured to use two reflectors for a laser cavity, and provide a gain by providing an optical gain unit between the two reflectors. In this case, a distance between the two reflectors is a cavity length. The optical communication system using the phase modulation method requires a laser light source having a very small line width, and the line width of the laser is decreased as the cavity length for configuring the laser is increased. Accordingly, the optical communication system using the phase modulation method requires a cavity laser device having a long cavity length.
Further, the optical communication system wishes to use a tunable laser device capable of flexibly using a wavelength band of a laser. In general, a wavelength tunable laser apparatus may be divided into a “single integrated wavelength tunable laser apparatus” in which an optical gain unit, a reflector, and elements for tuning a wavelength are formed on chips formed of the same medium, and an “external cavity wavelength tunable laser apparatus”, in which an optical gain unit and a wavelength tunable reflecting unit for tuning a wavelength are formed on chips formed of different media, respectively, and the two media are combined. Among them, the “external cavity wavelength tunable laser apparatus” has a longer cavity length than that of the “single integrated wavelength tunable laser apparatus”, and thus may have a considerably small line width according to the long cavity length. Accordingly, the “external cavity wavelength tunable laser apparatus” is more appropriate for a light source of the optical communication system using the phase modulation method.
However, the inventions in the related art adopt a method of separately manufacturing a package of a wavelength tunable laser apparatus and a package of a phase modulator, and then connecting the two packages through an optical fiber, so that there is a problem in that a volume of the wavelength tunable laser apparatus is increased, a structure of the package is complicated, and cost is increased.
Accordingly, in order to solve the problem and more improve performance of the wavelength tunable laser apparatus, development of a new technology has been demanded.